IMO OpenXcom (https://openxcom.org/) is a great example of what FOSS game engine recreation can bring to the table:
- multiplatform, compatible with modern systems
- definitive way to play X-COM:UFO Defense (aka UFO:Enemy Unknown) in 2019
- UI and graphics improvements
- ambitious mods and total conversions (like X-Piratez, Area 51, The Xcom Files)
- better AI and other bugfixes
- currently being expanded to run X-COM: Terror from the Deep
- completely different, unique gameplay experience not found in Firaxis XCOM remakes

Something like it. As I understand it mostly relies on OpenRA's modding capabilities, a bit like if OpenMW was post-1.0 enough that it could be used to make a decent emulation of Gothic (although KKnD is closer to RA1 than Gothic is to Morrowind).

I added TiaraCE, but it seems to be a dead project when I look at the github page. Did not feel like joining the discord channel just to confirm.

Project Tussle looks like a game clone more than an engine replacer. Correct me if I'm wrong. Same with Oolite.

Freeciv and freecol are game clones; new games that are heavily inspired by civilization. They are not engine replacers since they can't load the civilization assets. Great games though.

I updated Daggerfall Unity since they just became feature complete. Nice! Great work by them for sure. Daggerfall Unity is linked to under the "honorable mentions" header, since it technically isn't FOSS because of Unity. Still a great project worth checking out imo.

Really nice to see Daggerfall Unity (DFU) get the attention it deserves - even though it isn't technically FOSS, it's a fantastic project with a great community, and IMHO is already "the" way to play Daggerfall now, and will remain so in the future. Cheers!